1. Field of the Invention
The present invention relates to an optical pickup objective lens system and particularly, to an optical pickup objective lens system for use in an optical pickup device suitable for recording or replaying of high-density information in an optical recording medium.
2. Description of the Related Art
An optical pickup device is conventionally utilized widely for recording of information in an optical recording medium such as an optical disk, e.g., mainly, CD and DVD and for replying of the information recorded in the optical recording medium by applying a laser beam to the optical recording medium.
In such an optical pickup device, an increase in density of information recorded in an optical recording medium is required strongly, and this tendency has been further intensified in recent years.
To increase the recording density, it is necessary to use a laser beam having a shorter wavelength. For example, the wavelength of a laser beam used in the above-described CD is in a range of 780 to 790 nm, and a laser beam having a further shorter wavelength of 650 nm is used in DVD having a higher recording density. To realize a further increase in density, it is necessary to use a laser beam having a further shorter wavelength.
To accommodate to decrease in wavelength of a laser beam used with such an increase in density of the recorded information, it is demanded to enhance the optical performance of even an optical pickup objective lens system used in an optical pickup device, and an objective lens system which can accommodate to a laser beam having a shorter wavelength is desired.
In general, however, the wavelength of a laser beam is relied on a temperature, and the laser beam has a nature that its wavelength is varied slightly with a variation in temperature. In addition, the variation in wavelength of the laser beam occurs even due to a variation in applied voltage. The variation in wavelength of the laser beam also occurs, for example, when the voltage applied to a semiconductor laser is varied, such as when the operations of replaying and recording in an optical recording medium are changed over from one to the other.
Such a variation in wavelength of the laser beam exerts an influence to the refractive index of the optical pickup objective lens system, whereby a focusing point is displaced slightly, resulting in a problem that a chromatic aberration (longitudinal chromatic aberration) is generated.
Such problem of the chromatic aberration is more significant, as the recording density is further increased and the wavelength of the laser beam is more shortened. This is a large subject which must be solved in order to realize an optical pickup objective lens system using a laser beam having a shorter wavelength, for example, nearer to or smaller than 400 nm.
There are conventionally proposed optical pickup objective lens systems of a two-lens arrangement with a variation in wavelength of a laser beam taken into consideration, which are described in Japanese Patent Application Laid-open Nos.2000-90477, 2001-13406 and 2001-216678. Even in the lens system described in these Patent Applications, it is the actual circumstance that the problem of the chromatic aberration attendant on the variation in wavelength is not solved sufficiently.